ICs or LSIs, in a conventional way of producing an electronic circuit substrate, have been bonded onto a printed substrate through soldering of the electrodes thereof to the printed substrate. Soldering, however, does not enable effective bonding of the ICs or LSIs onto the printed substrate, and does not contribute to an increase in the packaging density of ICs or LSIs.
To solve the above problem, the ball grid array (BGA) technique has been developed which uses solder in a ball shape, so-called “solder balls”, to bond ICs or LSIs onto the substrate. In the case of using the BGA technique, solder balls mounted on a chip or a substrate are molten at high temperatures, thereby bonding the substrate to the chip. As a result, the production efficiency of electronic circuit substrates is improved. Moreover, electronic circuit substrates with improved packaging density of chips can be produced.
However, multi-layer substrates have been more used in recent years, and those multi-layer substrates, sensitive to the environmental conditions, are sometimes distorted, expanded or contracted to cause disconnection of the connection portions between substrates.
For example, when a solder ball is used to connect a semiconductor to a substrate, the solder ball undergoes stress because the semiconductor and the substrate are different in the linear expansion coefficient. As a result, the solder ball may have a crack to cause disconnection.
To solve such a problem, Patent Literature 1 discloses electroconductive microparticles each including a resin fine particle, an electroconductive metal layer containing a highly conductive metal on the surface of the resin fine particle, and a solder layer formed on the surface of the electroconductive metal layer. Such electroconductive microparticles can ease the stress applied thereto with soft resin fine particles. Since a solder layer is formed on the outermost surface of each electroconductive microparticle, electrodes are easily conductively connected to each other.
When electroconductive microparticles each with a solder layer formed thereon are mounted on electrodes of substrates, the electroconductive microparticles are placed on an electrode formed on one substrate. The solder layer is molten by reflowing, so that the electroconductive microparticles are immobilized on the electrode. Then, the substrates are arranged in such a manner that an electrode formed on the other substrate faces the electrode formed on the one substrate, and reflowing is performed to conductively connect the electrodes of the substrates.
If electroconductive microparticles each including a core particle and a solder layer formed on the surface of the core particle as disclosed in Patent Literature 1 are used for conductive connection of electronic circuit substrates having currently dominant copper electrodes, however, disconnection is likely to occur due to breakage of connection interfaces under application of an impact by dropping or the like. During the use of an electronic device, electronic components generate heat to increase the temperature inside the electronic device. After the use, the temperature inside the electronic device is lowered to ambient temperature. In this manner, the electronic device is subjected to repetitive heating and cooling, i.e., a “heat cycle”. Repetitive heat cycles also may cause breakage of connection interfaces between electrodes and electroconductive microparticles, resulting in disconnection.